1. Field of the Invention
The present invention relates to an image recording apparatus and an image recording method for recording an image on a recording medium by using a plurality of light modulator elements.
2. Description of the Background Art
Developed has been a diffraction grating type light modulator element which is capable of changing the depth of grating by alternately forming fixed ribbons and moving ribbons on a substrate with a semiconductor device manufacturing technique and sagging the moving ribbons relatively to the fixed ribbons. It is proposed that such a diffraction grating is used for an image recording apparatus in techniques such as CTP (Computer to Plate) as a switching element of light, since the intensities of normally reflected light and diffracted light are changed by changing the depth of grooves on the diffraction grating as above.
For example, a plurality of diffraction grating type light modulator elements provided in the image recording apparatus are irradiated with light, and then reflected light (zeroth order light) from light modulator elements in a state where the fixed ribbons and the moving ribbons are positioned at the same height from a base surface is guided to the recording medium and non-zeroth order light (mainly first order light) from light modulator elements in a state where the moving ribbons are sagged is blocked, to achieve an image recording on the recording medium.
Japanese Patent Application Laid Open Gazette No. 2004-4525 (Document 1) discloses a technique correcting the timing of transition between ON and OFF states of a light modulator element in such an image recording apparatus to correct asymmetry between transition from the OFF state to the ON state and transition from the ON state to the OFF state, difference in characteristics of each photosensitive material, and positional shifts of writing regions caused by difference in length or position in a scan direction of irradiation regions of light modulator elements.
Japanese Patent Application Laid Open Gazette No. 2001-150730 (Document 2) discloses a technique for forcedly interrupting light from liquid crystal shutters during a transient response period with a mechanical shutter to remove effects of unevenness in exposure by the liquid crystal shutters and difference in transient responses of the liquid crystal shutters in an image forming apparatus.
In the diffraction grating type light modulator elements, amounts of light derived from light modulator elements slightly vary even if all light modulator elements are made the ON state, because there are nonuniformity of light from a light source and differences of characteristics among the light modulator elements. Since such a variation causes striped moire in writing an image of a fine pattern, it is important that a light amount from each light modulator element is corrected to be uniform by controlling the height of the moving ribbons in the ON state.
However, in a case where the heights of the moving ribbons in the ON state are different in light modulator elements, since the amounts of movement of the moving ribbons between the ON and OFF states are not uniform, a time from when a signal instructing ON is inputted to a driving element of the light modulator element to when the light modulator element is actually brought into the ON state (hereinafter, referred to as “rise time”) and a time from when a signal instructing OFF is inputted to the driving element of the light modulator element to when the light modulator element is actually brought into the OFF state (hereinafter, referred to as “fall time”) are different from those in another light modulator element.
In Document 1, the unevenness of the rise times and the fall times, that is, temporal unevenness in movement of the light modulator elements with respect to the signal instructing driving is not taken into consideration. Consequently, when a line with a constant width extending in a sub scan direction is written, the line width slightly changes. Though Document 2 discloses a technique for removing effects of unevenness in the transient response at the rise in liquid crystal shutters by using the mechanical shutter, since light is blocked mechanically, the mechanism of the apparatus becomes complicated and speed-up of writing is prevented. Further Document 2 also discloses a technique of controlling timing at the fall to make the exposure amount uniform, however, unevenness in the transient response at the fall is not considered.